In an optical receiver circuit used in an optical communication system or the like, an optical signal from an optical fiber is converted to a current by a photo diode, and then the current is converted to a voltage signal by the optical receiver preamplifier. If a signal transmission distance through an optical fiber is long, a transmission loss in the fiber becomes large, and a current signal having a very small amplitude is input from the photo diode to the optical receiver preamplifier. In contrast, if a signal transmission distance through an optical fiber is short, a transmission loss in the fiber becomes small. A current signal having a large amplitude is input from the photo diode to the optical receiver preamplifier. Thus, current signals having various different amplitudes ranging from a small amplitude to a large amplitude are input to the optical receiver preamplifier.
Therefore, when a current-voltage conversion gain (transimpedance gain) of the optical receiver preamplifier is increased to detect a small signal, at a time of inputting a large signal, an output voltage is saturated and a waveform is distorted and the signal can not be precisely detected. On the other hand, when the current-voltage conversion gain is reduced to lower saturation of the output voltage, at a time of inputting a small signal, the small signal can not be sufficiently amplified and detection of a small signal is difficult.
To solve the above-described problems, in an optical receiver signal preamplifier disclosed in Patent Reference 1, a diode is connected in parallel to a feedback resistor (RF) connected between input and output terminals of an inverting amplifier. Thus, a voltage drop in the feedback resistor can be cramped, so that saturation of an output voltage can be suppressed. Moreover, an optical receiver preamplifier disclosed in Patent Reference 2, a transistor is connected to the feedback resistor in parallel and an impedance of the transistor is modulated. Thus, a resistance value of a feedback resistor, i.e., a current-voltage conversion gain is varied to prevent saturation of an output voltage.
(Patent Reference 1) Japanese Laid-Open Publication No. H03-060208
(Patent Reference 2) Japanese Laid-Open Publication No. H09-232877